1. Field of the Invention
The present invention generally relates to the field of radio telecommunications, particularly, albeit not limitatively, for broadcast transmissions, such as TeleVision (TV), multimedia or, generally, information broadcasting. More specifically, the invention concerns a method and a system for obtaining an essentially omnidirectional radioelectric coverage of an area of interest through a combination of two or more sector antennas.
2. Description of the Related Art
In broadcast radio telecommunications systems, like for example the DVB-T (Digital Video Broadcasting-Terrestrial) system for the diffusion of digitally-coded TV contents, the TV signal is typically broadcast across a geographic area, intended to be covered by the service, exploiting transmitting sites (also known as “transmitters”) having transmitting antennas; the transmitters are properly deployed depending on the characteristics (e.g., the morphology, particularly the orography) of the territory of the area of interest.
Regardless of the territory characteristics, the radioelectric coverage of the area of interest should be as uniform as possible; to this purpose, the transmitting sites are typically installed at elevated locations, like for example on the top of hills or mountains, or on high towers. Receiving antennas are, on their side, installed on the roofs of houses or buildings. In this way, the propagation of the signal from the transmitting antennas to the receiving antennas is, with a high likelihood, in “Line Of Sight” (“LOS”); the LOS signal propagation is regarded as the best condition for the signal not to be degraded due to the presence of obstacles on the propagation channel.
Recent digital multimedia contents broadcast systems like the DVB-H (DVB-Handheld) have radioelectric coverage and mobility requirements that make the process of planning of the transmitter network more similar to the planning of a mobile telephony network than to that of a TV broadcast system.
The DVB-H system as a matter of fact provides for the wide-band digital signal to be received by mobile communications terminals (like DVB-H mobile phones) which experience a radio propagation channel strongly affected and degraded by the signal propagation environment, particularly when the user is inside buildings in dense urban areas; in this situation, the signal propagation from the transmitter to the receiver is not in LOS, and is thus defined as “No LOS” (“NLOS”).
Additionally, the receivers in DVB-H terminals have performance that are likely to be lower than the performance of the receivers of the mobile phones, particularly under the respects of the sensitivity and the antenna efficiency, due to the required operating bandwidth and the fact that the wavelength of the radio carrier of the DVB-H signal is by far bigger than the practical size of a mobile communications terminal.
The mobility of the terminals further requires that the modulation schemes, the signal encoding and decoding algorithms and the equalization techniques directed to recovering the different signal contributions deriving from multiple signal propagation paths be sufficiently robust, so as to support the service continuity when the mobile terminal moves across the area intended to be covered by the service. Due to this, the requirements on the radioelectric coverage and the signal-to-interference ratio are more stringent compared to digital TV broadcast systems which, like the DVB-T, do not support mobility of the receivers.
A consequence of all this is that the signal broadcast network cannot in general be made up of relatively few transmitter sites, located at relatively elevated positions: the broadcast network needs additional transmitters having lower coverage and deployed across the territory with a density similar to that of the transmitters of a mobile telephony network.
In radio telecommunications systems like mobile telephony networks and radio and/or TV broadcast networks the coverage of the geographic area of interest is typically achieved by means of signal transmitters coupled to omnidirectional and/or sector antennas. Omnidirectional antennas are capable of irradiating the signal substantially uniformly around them, whereas sector antennas are capable of irradiating the signal in angular sectors whose width depends on the electromagnetic features of the antennas.
In radio and TV broadcasting the omnidirectional coverage is often achieved by combining a number of sector antennas fed by the same signal through a properly designed signal distribution network; the reason for this stems from the reflectivity of the trellises sustaining the antennas. Using a combination of sector antennas also gives to the network planner more degrees of freedom in the signal irradiation scheme (for example, it allows shaping the radiation diagrams to take into account the territory morphology). The technique of reconstructing an omnidirectional radiation diagram through the combination of equidistant sector antennas which are equally fed with in-phase signals requires that the antennas are spaced apart of fractions of the wavelength of the irradiated signal. Practically, the sector antennas that are combined to give an omnidirectional coverage are placed in such a way as to form a regular polygon and are spaced apart from one another of fractions (typically, one half) of the signal wavelength. The shape of the radiation diagram will depend on the relative positions of the antennas, on their radiation characteristics and on the phase and amplitude relationships between the signals that feed the different antennas.
The above technique can be adopted in traditional radio and TV broadcasting systems, where the antennas are placed on hills, mountains or trellises, where there are not problems of space and the sector antennas that are combined to emulate an omnidirectional antenna can be placed at the suitable distance from each other.
However, in radio and TV broadcasting systems that have to support mobility of the receivers, like the DVB-H, which require that the signal be broadcast also by transmitters spread across the territory, it would be desirable to use, for the installation of the antennas, sites of an existing mobile telephony network. These sites, conceived for supporting sector antennas, are architecturally not suitable for placing omnidirectional antennas or for placing the sector antennas sufficiently close to each other so as to emulate the radiation diagram of an omnidirectional antenna. For example, in a trellis of a tri-sector site of a mobile telephony network three sector antennas are placed at the vertexes of an equilateral triangle having sides of length equal to some wavelengths of the DVB-H radio-frequency signal that should be transmitted. Thus, the technique exploited in conventional, radio and/or TV broadcasting networks for achieving an omnidirectional radioelectric coverage cannot be relied upon.
There has been some work in the area of transmitting a downlink signal from a base station to one or more mobile terminals with the goal of achieving a desirable radiation diagram that can be in the more general case omnidirectional. For example, U.S. Pat. No. 6,185,440 presents a possible solution through the exploitation, at the base station side, of an array of antenna elements with suitable signal processing operations (weighting) on the signals transmitted by the different elements of the antenna array. In the particular case of U.S. Pat. No. 6,185,440 the generation of the required radiation diagram can be achieved by sequentially repeating the transmission of the downlink signal in such a way that, at each repetition, a different set of weighting coefficients is employed, until when the overlapping of the radiation patterns employed at each repetition generates the required radiation diagram.